A four-stroke engine, commonly found in inboard and outboard engines, operates through four distinct strokes that complete two full revolutions of the crankshaft.

The four strokes are intake, compression, power, and exhaust. Here's a breakdown:

1. Intake Stroke: The piston moves downward, creating a vacuum in the cylinder. This opens the intake valve, allowing a fuel-air mixture to enter the cylinder from the intake manifold.

2. Compression Stroke: The intake and exhaust valves close, and the piston moves back upward, compressing the fuel-air mixture. This compression increases its pressure and temperature, preparing it for combustion.

3. Power Stroke: When the piston reaches the top of the cylinder, the compressed fuel-air mixture is ignited by a spark plug (in gasoline engines) or by compression (in diesel engines). The resulting explosion forces the piston back down, generating the power that drives the crankshaft.

4. Exhaust Stroke: As the piston moves back up, the exhaust valve opens, and the remaining exhaust gases from the combustion process are expelled from the cylinder into the exhaust system.